1. Field of the Invention
This invention relates to a plasma display panel (PDP) using a radio frequency discharge, and more particularly to a driving circuit for a radio frequency PDP that is capable of effectively matching an impedance between a radio frequency signal generator and the plasma display panel.
2. Description of the Related Art
Recently, a plasma display panel (PDP) feasible to the fabrication of large-scale panel has been available for a flat panel display device. The PDP includes discharge cells corresponding to color pixels of matrix type and controls a discharge interval of each discharge cell to display a picture. More specifically, after the PDP selected discharge cells to be displayed by an address discharge, it allows a discharge to be maintained in a desired discharge interval at the selected discharge cells. Thus, in the discharge cells, a vacuum ultraviolet ray generated during the sustaining discharge radiates a fluorescent material to emit a visible light. In this case, the PDP controls a discharge-sustaining interval, that is, a sustaining discharge frequency of the discharge cells to implement a gray scale required for an image display. As a result, the sustaining discharge frequency becomes an important factor for determining the brightness and a discharge efficiency of the PDP. For the purpose of performing such a sustaining discharge, a sustaining pulse having a frequency of 200 to 300 kHz and a width of about 10 to 20 xcexcs has been used in the prior art. However, the sustaining discharge is generated only once at a extremely short instant per the sustaining pulse by responding to the sustaining pulse; while it is wasted for a step of forming a wall charge and a step of preparing the next sustaining discharge at the remaining major time. For this reason, the conventional three-electrode, face-discharge, and AC PDP has a problem in that, since a real discharge interval is very short in comparison to the entire discharge interval, the brightness and the discharge efficiency become low.
In order to solve such a problem of low brightness and low discharge efficiency, we has suggested a method of utilizing a radio frequency discharge employing a radio frequency signal of hundreds of MHz as a display discharge. In the case of the radio frequency discharge, electrons perform an oscillating motion by the radio frequency signal to sustain the display discharge in a time interval when the radio frequency signal is being applied. More specifically, when a radio frequency signal with a continuously alternating polarity is applied to any one of the two opposite electrodes, electrons within the discharge space are moved toward one electrode or the other electrode depending on the polarity of the voltage signal. If the polarity of a radio frequency voltage signal having been applied to the electrode before the electrons arrive at the electrode is changed when electrons are moved into any one electrode, then the electrons has a gradually decelerated movement speed in such a manner to allow their movement direction to be changed toward the opposite electrode. The polarity of the radio frequency voltage signal having been applied to the electrode before the electrons within the discharge space arrive at the electrode is changed as described, so that the electrons make an oscillating motion between the two electrodes. Accordingly, when the radio frequency voltage signal is being applied, the ionization, the excitation and the transition of gas particles are continuously generated without extinction of electrons. The display discharge is sustained during most discharge time, so that the brightness and the discharge efficiency of the PDP can be improved. Such a radio frequency discharge has the same physical characteristic as a positive column in a glow discharge structure.
FIG. 1 is a perspective view showing the structure of a discharge cell of the above-mentioned radio frequency PDP employing a radio frequency discharge. In FIG. 1, the discharge cell 26 includes radio frequency electrodes 12 provided on an upper substrate 10, data electrodes 16 and scanning electrodes 20 provided on a lower substrate 14 in such a manner to be perpendicular to each other, and barrier ribs 22 provided between the upper substrate 10 and the lower substrate 14. The radio frequency electrodes 12 apply a radio frequency signal. The data electrodes 18 apply a data pulse for selecting cells to be displayed. The scanning electrodes 20 are provided in opposition to the radio frequency electrodes 12 in such a manner to be used as opposite electrodes of the radio frequency electrodes 12. Between the data electrodes 18 and the scanning electrodes 20 is provided a dielectric layer 18 for the charge accumulation and the isolation. The barrier ribs 22 shut off an optical interference between the cells. In this case, the barrier ribs 22 are formed into a lattice structure closed on every side for each discharge cell so as to isolate the discharge space. This is because it is difficult to isolate a plasma for each cell unlike the existent face-discharge due to the opposite discharge generated between the radio frequency electrodes 12 and the scanning electrodes 20. Also, the barrier ribs 22 have a more enlarged height than the conventional barrier ribs for the sake of providing a smooth radio frequency discharge between the scanning electrodes 20 and the radio frequency electrodes 12. A fluorescent material 24 is coated on the surface of the barrier rib 22 to emit a visible light with an inherent color by a vacuum ultraviolet ray generated during the radio frequency discharge. The discharge space defined by the upper substrate 10, the lower substrate 14 and the barrier ribs 22 is filled with a discharge gas.
As shown in FIG. 2, the discharge cells 26 having the configuration as described above are positioned at each intersection among data electrode lines X1 to Xm, scanning electrode lines Y1 to Yn and radio frequency electrode lines RF. In FIG. 2, the data electrode lines X1 to Xm consist of the data electrodes 16 of the discharge cells 26. The scanning electrode lines Y1 to Yn consist of the scanning electrodes 20, and the radio frequency electrode lines RF consist of radio frequency electrodes 12. A driving method of the discharge cell 26 of FIG. 1 will be described below. A data pulse DP is applied to the address electrode 16 and a scanning pulse SP is applied to the scanning electrode 20 to generate an address discharge. By this address discharge, charged particles are produced at a discharge space. The charged particles make a radio frequency discharge with the aid of a radio frequency pulse RFS applied to the radio frequency electrode 12 and a center voltage Vc of a radio frequency voltage applied to the scanning electrode 20 constantly. In this case, an ultraviolet ray generated by the radio frequency discharge radiates a fluorescent material 24 to emit a visible light. When an erasure pulse is applied to the scanning electrode 20, the charged particles becomes distinct to stop the radio frequency discharge.
In order to cause a radio frequency discharge from the radio frequency PDP including the discharge cells as described above, a radio frequency signal having a sufficient power must be applied to radio frequency electrode lines RF of the panel. A conventional PDP driving circuit including a radio frequency driving circuit for obtaining this purpose is shown in FIG. 3.
Referring to FIG. 3, the conventional radio frequency PDP driving circuit includes an analog to digital (A/D) converter 30 for converting an input analog image signal into a digit signal, an image signal processor 32 for converting the digit signal from the A/D converter 30 into a bit signal to re-arrange the same for each bit, a data driver 34 for outputting a driving signal according to a data signal inputted from the image signal processor 32 to data electrode lines of a PDP 42, a radio frequency generator 36 for generating a radio frequency signal, an amplifier 38 for amplifying and outputting the radio frequency signal from the radio frequency generator 36, an impedance matcher 40 for matching impedance of the amplifier 38 with that of the PDP 42, and a scanning driver for driving scanning electrode lines of the PDP 42. The A/D converter 30 converts an input analog image signal into a digit signal and outputs it. The image signal processor 32 converts the digit signal inputted from the A/D converter 30 into a bit signal to re-arrange the bit signal for each bit in compliance with a driving of the PDP 42 and output it. The data driver 34 applies a driving signal according to an image data inputted from the image signal processor 32 to the data electrode lines of the PDP 42. The radio frequency generator 36 generates a radio frequency signal to apply it to the amplifier 38. The amplifier 38 amplifies the radio frequency signal from the radio frequency generator 36 into a power enough to cause a radio frequency discharge to output the same to the impedance matcher 40. The impedance matcher 40 matches impedance of the amplifier 38 with that of the panel 42 to apply a maximum power of radio frequency signal to the radio frequency electrode lies of the panel 42. The scanning driver 44 applies a scanning signal to the scanning electrode lines of the PDP 42. Basically, the panel 42 has a capacitance. In this case, if a radio frequency discharge is generated at the panel 42, then a phenomenon of increasing a capacitance of the panel occurs because a sheath is produced at a radio frequency electrode causing the radio frequency discharge and a scanning electrode to narrow a distance between the two electrodes determining a capacitance value. Thus, impedance of the panel is reduced to absorb (or pass) the radio frequency signal within the panel 42, so that a power of the radio frequency signal applied to the panel 42 is reduced. Therefore, the impedance matcher 40 for matching impedance between the radio frequency amplifier 38 and the panel 42 is one of important elements in the radio frequency driving circuit. This is because a maximum power of radio frequency signal is applied to the panel 42 when impedance of the radio frequency amplifier 38 becomes equal to that of the panel 42 so that the panel 42 can be stabbly driven. Generally, an incident wave and a reflective wave co-exists in the radio frequency driving circuit. In real, a power superposed with an incident wave and a reflective wave is applied to the panel 42. Accordingly, the application of a maximum power of radio frequency signal by the impedance matching at the impedance matcher 40 means that an incident wave is applied to the panel 42 as it is, with making a minimum reflective wave.
To this end, as shown in FIG. 4, the impedance matcher 40 includes a first capacitor C1 connected between a first node N1 at the output terminal of the amplifier 38 and a ground, and a serial connection of a second capacitor C2 and an inductor L between the first node N1 and the input terminal of the panel 42. By values of the first and second capacitors C1 and C2 and the inductor L, impedance matching between the amplifier 38 and the panel 42 is made. In this case, the first and second capacitors C1 and C2 and the inductor L are fixedly designed to have optimum values depending on impedance of the panel 42 and a characteristic of the entire system in the PDP.
One of important parameters in making an impedance matching at such an impedance matcher 40 is a length of a radio frequency supply line connected to the impedance matcher 40 and each radio frequency electrode line of the panel 42. The PDP has being developed for the purpose of providing a large-scale display of more than 40 inch that requires a length of more than at least 50 to 60 Cm on the basis of a distance between the top and the bottom of the panel. In other words, a top-to-bottom length of the panel in the large-scale PDP requires a length of tens of to hundreds of Cm. In such a PDP having the panel length of tens of to hundreds of Cm, however, a length difference of several to tens of Cm is generated between the radio frequency supply lines connected to the matcher 40 and each radio frequency electrode lines of the large-scale panel 42. Since impedance is changed due to such a length difference of the radio frequency supply lines to apply radio frequency signals with a different power to each radio frequency electrode line, the PDP fails to make a stable radio frequency discharge.
Referring now to FIG. 5, there is shown a panel 42 including radio frequency electrode lines RF1 to RFn connected commonly to the conventional impedance matcher 40. As seen from FIG. 5, when a single of impedance matcher 40 for making an impedance matching exists, the length of supply lines a connected to each radio frequency electrode line RF1 to RFn becomes different wherever the impedance matcher 40 is positioned. Particularly, when the panel 42 is made into a large-scale, a length difference in each supply line a becomes more than several to tens of Cm. If the radio frequency supply lines a have such a difference, particularly, a difference of more than tens of Cm, then a considerably large impedance difference is generated. This results in an impedance difference being generated between the radio frequency electrode lines RF1 to RFn. In particular, when the impedance matcher 40 is located at the center in the upward and downward direction of the panel 42, a length difference between the supply line a connected to the (n/2)th radio frequency electrode line RFn/2 positioned at the center of the panel 42 and the supply line a connected to the first or nth radio frequency electrode line RF1 or RFn, that is, an impedance difference therebetween is particularly large. Since impedance between the radio frequency electrode lines RF1 to RFn becomes different due to such a length difference of the radio frequency supply lines a to supply a different power of radio frequency signal in spite of the same load, that is, the same radio frequency electrode, a stable radio frequency discharge can not be obtained. Also, since a different magnitude of radio frequency signal is applied to each radio frequency electrode line RF1 to RFn due to an impedance difference according to a length difference of the radio frequency supply lines a, an intensity of light occurred by discharge becomes non-uniform to distort a picture. These problems become more serious as the radio frequency is higher and the size of the panel 42 is larger, the more is serious.
Meanwhile, in order to make a matrix driving of the panel 42, each discharge cell must be independently driven and, at the same time, a radio frequency signal with an constant level must be applied to the panel 42. When a radio frequency signal is reduced by discharge cells generating a radio frequency discharge, however, a sufficient power of radio frequency signal is not applied to discharge cells in which a radio frequency discharge is to be generated after that time, so that the radio frequency discharge may not cause in the discharge cells.
More specifically, impedance of the panel 42 is varied by inputted signal to be displayed. For instance, it is assumed that, when only image signals having a black level are inputted, that is, when a radio frequency discharge does not occur, impedance of the panel 42 is the smallest value ZMIN. Also, it is assumed that, when image signal expressing a white level only are inputted, that is, when a radio frequency discharge is generated continuously during one frame at the entire panel 42, impedance of the panel 42 is the largest value ZMAX. In this case, it can be said that all of the impedance values which the panel 42 can have correspond to a value between ZMIN and ZMAX. Accordingly, a method of assuming impedance of the panel 42 to be a intermediate value between ZMIN and ZMAX in correspondence with variable impedance of the panel 42 and matching impedance of the radio frequency amplifier 38 with that impedance has been applied to the conventional impedance matcher 40.
However, since the conventional impedance matcher 40 can not cope with an impedance variation of the panel 42 adaptively, it is difficult to apply a maximum power of radio frequency signal to the panel 42. First of all, it is important to supply a constant radio frequency signal for a momentarily changing image signal when a moving picture is displayed on the panel 42. Since the conventional impedance matcher 40 having a fixed impedance value fails to adaptively cope with an impedance variation of the panel 42 so that a maximum power of radio frequency signal can not be applied to the PDP 42, however, it is difficult to generate a stable radio frequency discharge at the panel 42.
Accordingly, it is an object of the present invention to provide a radio frequency driving circuit wherein an impedance difference between radio frequency electrode lines caused by a length difference of radio frequency supply lines is compensated to make a stable driving of a radio frequency PDP.
A further object of the present invention is to provide a radio frequency driving circuit that is capable of adjusting to impedance of a PDP varied in accordance with a brightness level of an image signal so as to supply a maximum power of radio frequency signal.
In order to achieve these and other objects of the invention, a radio frequency PDP driving circuit according to one aspect of the present includes a plurality of impedance matching means, being independently connected to each group of radio frequency electrodes, to match impedance of input and output terminals, said radio frequency electrodes being divided into a plurality of groups.
A radio frequency PDP driving circuit according to another aspect of the present includes impedance matching means for varying an impedance matching value in accordance with an input control signal to match impedance between an input terminal to which a radio frequency signal is applied and the panel; and control means for generating said control signal to set an impedance matching level in accordance with a brightness level of an input image signal and to control an impedance matching value of the impedance matching means in accordance with the impedance matching level.